OBSERVATIONS OF TIDAL FLUX BETWEEN A SUBMERSED AQUATIC PLANT STAND AND THE ADJACENT CHANNEL IN THE POTOMAC RIVER NEAR WASHINGTON, DC

Dye injection studies and direct velocity and water-level measurements were made in macrophyte stands and adjacent channels in order to obse rve the effects of the macrophyte stand on flow and mass exchange in t he tidal Potomac River. During the summer, dense stands of submersed a quatic plants cover most shoals <2 m deep. Continuous summertime water -level records within a submersed aquatic plant stand and in the adjac ent channel revealed time-varying gradients in water-surface elevation between the two areas. Water-level gradients are created by differing rates of tidal water-level change in vegetated and unvegetated areas. Results were consistent with the idea that on a rising tide the water was slower to enter a macrophyte stand, and on a falling tide it was slower to leave it. Differences in water elevation between the stand a nd the open channel generated components of velocity in the stand that were at right angles to the line of flow in the channel. Seasonal dif ferences in flow speed and direction over the shoals indicate substant ial differences in resistance to flow as a result of the vegetation.